A polyimide film has been selected for various productive uses, which requires at least some extent of thermal stability, satisfactory electrical and mechanical characteristics. In addition to these characteristics, a polyimide film further having a desirable transparent characteristic has been created and widely adopted as an oriented film for liquid crystal display material, waveguide material and solar battery protection film. To be adopted for this purpose, the polyimide film has to have a desirable transparency. However, a polyimide film is often turned (changed to) yellow or brown due to thermal degradation resulting from severe thermal history in the depositing process. The coloring due to heat degradation damages a function of a display device, as it darkens the view, and therefore it had been considered unacceptable for the use of an oriented film of liquid crystal or the like.
In order to clear away this defect, various polyimide films have been created to realize a polyimide film causing low coloring and having high transparency. This prior art of polyimide films has been developed with a series of studies focusing on the change in color due to thermal degradation of a transparent polyimide film. More specifically, for example, Matuura et al. disclosed a structure effective for creating a polyimide film having superior transparency. The structure substitutes C—H bonding of a material monomer for C—F bonding, in other words, includes a fluorine substituent in the material monomer. However, it generally costs highly to use the material monomer including a fluorine substituent.
Further, in imidation by heating polyamic acid at high temperature (for example, at or higher than 400° C.) so as to express toughness of the film, or imidation by a chemical cure method which uses acid anhydride such as acetic anhydride, and tertiary amine such as picoline, it is experientially known that the obtained polyimide is colored due to thermal degradation or chemical reaction (the imidation causes bad effect for the color). However, when the polyamic acid is heated at relatively low temperature in order to prevent the foregoing problem, sufficient strength is not shown in the obtained film.
An object of the present invention is to provide a polyimide film essentially transparent and colorless, and has sufficient toughness to prevent occurrences of a crack or a break when the film is folded and creased by hand.